1. Field of the Invention
The present invention relates to a method and device for the generation of a random signal. The invention can be applied especially to the field of digital-analog conversion and analog-digital conversion using a random system of this kind.
It can be applied for example in the field of radar techniques or in that of instrumentation or again in the field of communications.
Conversion devices, whether digital-analog or analog-digital conversion devices, are very widely used in many systems, and their performance characteristics are generally an essential point of these systems as is illustrated in direct digital synthesis.
Direct digital synthesis is a technique of frequency synthesis in which the samples of a signal to be generated are elaborated in the form of digital values and these samples are converted into analog signals by means of a digital-analog converter. The signal synthesizers obtained by this technique are highly attractive in terms of volume, weight and energy consumption because they benefit from large-scale integration. The other advantages especially are very high resolution and very low switching time from one frequency to another. However, the passage of a useful signal into the digital-analog converter is accompanied by the creation of spurious signals due to the non-linearities of these converters. These non-linearities designate the fact that the stairs or steps of the transfer function of the digital-analog converter are not equal in height and that the transition between steps produces uneven phenomena.
The same problem can be found in applications based on analog-digital converters where the passage of the signals into these converters is accompanied here too by the creation of spurious signals due to the non-linearities.
2. Description of the Prior Art
There are known ways in the prior art of adding a random signal into the useful signal, before its passage into the converter, in order to reduce the level of the parasite signals by reducing the effect of the above-mentioned non-linearities of the converter. This random signal is commonly called xe2x80x9cditherxe2x80x9d. The useful signal generally has a limited bandwidth and the clock frequency of the system, this system being for example a digital synthesizer, is generally greater than this band. This leaves a vacant spectral space in which to place the random signal.
To obtain full efficiency, this random signal must have certain characteristics. First of all, its spectrum must be controlled so that it does not encroach on the band of the useful signals. Secondly, it appears that the quality of the linearization of the converters depends on the histogram of the temporal amplitudes of the random signal. For example, a Gaussian relationship produces a linearization that is not as good as the one obtained by a rectangular relationship. There is therefore real advantage in being able to control both the spectrum and the histogram for the random signal.
There are known methods used to obtain a random signal with a given spectral envelope. Methods are also known to obtain a random signal with a given law of distribution of the amplitudes. These methods are described especially in works on the computation of probabilities such as, for example, J. Maurin, xe2x80x9cSimulation dxc3xa9terministe du hazardxe2x80x9d (Deterministic simulation of random processes), Editions Masson.
The patent FR 2 783 374 by the present applicant teaches a method and device for the generation of a random signal. It describes a method for the construction of a random signal in which the spectral envelope and the law of distribution of the temporal amplitudes are imposed simultaneously. To this end, the method implements a sequence of four signal-processing steps or operations in which the repetition of a part among them, especially the steps 3 and 4, make the parameters of the random signal converge toward the desired distribution. The iteration of the steps makes it possible to gradually approach the fixed distribution law and then to correct the spectral envelope.
Despite all its efficiency, this iterative method is not adapted to all types of computation, especially to the real-time computation of the random signal. It implies the use of various non-linear functions to restore the histogram aimed at in each iteration.
The idea of the invention is based on a novel approach enabling the real-time computation of a random signal with a predetermined spectral envelope and a histogram of amplitudes close to a rectangular distribution, namely any equidistributed relationship.
Hereinafter in the invention, the term xe2x80x9cuseful signalxe2x80x9d designates the signal to be converted, without distortion, by a DAC or an ADC. To this end, the random signal or noise that is generated by the device according to the invention is added to this useful signal so as to linearize the transfer characteristic of the DAC or ADC.
An object of the invention is a method for the generation of a random signal. The method comprises at least the following steps:
A first step (a) for the generation of a pseudo-random signal,
a second step (b) for the filtering (F1) of the signal coming from the step (a) to obtain a signal x(t) having a predetermined spectral envelope H(f),
a third step (c) in which a non-linear function g is applied to the signal x(t) so as to form a signal y(t) and create overshoots on the edges of the histogram of the signal y(t),
a fourth filtering (F2) step (d) used to smoothen the overshoots of the histogram of the signal y(t), compensate for the effect of the non-linearity and carry out an additional filtering at (F1).
The overshoots are more or less pronounced, depending especially on the shape of the final histogram.
According to one embodiment, the non-linear function is, for example, a function with facets Di and the number of the segments and the ratio of the slopes of the different segments are chosen as a function of the histogram obtained in the filtering step F1.
The pseudo-random signal is, for example, a white noise.
An object of the invention is also a device for the implementation of the above-described method comprising for example at least the following elements:
a) means to generate a pseudo-random signal,
b) means (F1) to filter the pseudo-random signal in order to obtain a signal x(t) having a predetermined spectral envelope H(f),
c) a device adapted to generating a non-linear function to form a signal y(t) from the signal x(t) having a Gaussian type of histogram, the histogram of this signal y(t) being of a rectangular type with overshoots,
d) means (F2) adapted to smoothening the overshoots of the histogram of the signal y(t), compensating for the effect of non-linearity and making an additional filtering at (F1).
The signal generated is, for example, a white noise.
The invention in particular has the following advantages:
it improves the non-linearities of the analog-digital converters or digital-analog converters
it is applicable to many systems,
it is economical and simple in its implementation